This is a renewal application designed to determine the role of the human specific Na+/H+ exchanger isoform, NHE-2, in Na+ absorption by the human small intestine. The P.I. hypothesizes that the promoters for human NHE-2 and NHE-3 isoforms contain some distinctive regulatory elements to explain their unique expression patterns and regulation. To address this hypothesis, the P.I. is planning to address two specific aims. The first aim is designed to clone the promoter for the human NHE-2 gene and characterize its regulatory elements. He will utilize the classical methods to identify the transcription initiation site and determine the regulatory elements contained within the promoter region. He will then determine the DNA-protein interactions in models of differentiation and development utilizing mobility shift analysis. The roles of the promoter sequences for NHE-2 and NHE-3 and their differential expression in epithelial cells will be determined using anti-sense oligonucleotide techniques. The second specific aim is designed to determine the structure function relationship of NHE-2. To achieve this aim, the P.I. is planning to use site-directed mutagenesis and truncation mutants to determine the role of the specific transmembrane domains essential for Na+ transport function, H+ modifier and for amiloride binding sites. He will also identify the specific cytoplasmic domains for NHE-2 regulation by protein kinases and growth factors. Finally, he will determine the protein domains responsible for protein targeting and polarized distribution of NHE-2 in intestinal cells.